We studied the dispersion of Northern Waterthrushes (Seiurus novaboracensis) in southwestern Puerto Rico during four nonbreeding seasons, 1996–1999. Densities were high (up to 13 birds/ha) on a 3-ha mature black mangrove (Avicennia germinans) study plot, but were significantly lower during periods of high water levels. Individuals exhibited site fidelity within and between seasons. Feeding areas were small (mean = 0.074 ha ± 0.041 SD) and there was considerable overlap tolerated among conspecifics. Waterthrush density decreased when water submerged their primary foraging substrate: woody debris and pneumatophores. Interannual returns were similar to other Neotropical migrants (mean = 50%) but site persistence was low due to periodic flooding. In September 1998, hurricane Georges flooded the plot and blew down >90% of the black mangrove trees. This drastic habitat alteration was followed by a drastic decline in waterthrushes using the study area. Individuals left feeding areas for overnight roost sites in red mangroves (Rhizophora mangle). This latter finding, coupled with site fidelity and high return rates concurrent with low site persistence, suggests that waterthrushes exhibit high plasticity in their use of habitat during the nonbreeding season, but may rely upon mangroves for overwinter survival.